1. Field of the Invention
This invention relates to a disposable diaper excelling in resistance to pressure and an absorbent resin excelling in resistance to pressure. Further, this invention relates to an absorbent resin preeminently excelling in affinity for aqueous liquids and exhibiting prominently improved speed of absorption and ratio of water absorption capacity under no pressure and under pressure as compared with the conventional equivalents and a method for the production thereof.
2. Description of Related Art
In recent years, absorbent resins capable of absorbing some ten to some hundred times their own weights of water have been developed and have been found utility in various applications requiring absorption and retention of water such as in the field of hygienic materials including sanitary goods and disposable diapers, the field of agriculture and horticulture, the field of foodstuffs requiring retention of freshness, and the field of industrial products requiring protection against dew formation and refrigeration.
The absorbent resins of this class have been known in various types such as, for example, a hydrolyzed starch-acrylonitrile graft polymer, neutralized starch-acrylic acid graft polymer, saponified vinyl acetate-acrylic ester copolymer, hydrolyzed acrylonitrile copolymer or the product of cross-linkage thereof, hydrolyzed acrylamide copolymer or the product of cross-linkage thereof, self-cross-linking polyacrylic acid salts, and partially neutralizing cross-linked polyacrylic acid salts.
The absorbent resin has the necessary quality thereof varied by the kind of use. As the characteristics which are required by an absorbent resin intended for such hygienic materials as disposable diaper, for example, high ratio of water absorption capacity, high speed of absorption, large capacity for permeation, etc. to be manifested to an aqueous liquid under pressure may be cited. EP 443627, for example, discloses an absorbent structure which uses an absorbent resin excelling in ratio of water absorption capacity and speed of absorption under pressure in a concentration of not less than 60 wt %. The properties of ratio of water absorption capacity and speed of absorption do not always exhibit a positive correlation. Particularly, the ratio of water absorption capacity and the speed of absorption under pressure betray antinomy. It is, therefore, difficult to have these two properties simultaneously improved.
Addition to surface area, for example, has been tried with a view to enabling an absorbent resin to acquire a heightened speed of absorption. Attempts have been made to decrease the particle diameter of the absorbent resin or fabricate the absorbent resin in the form of granules or scales. Generally when the absorbent resin is given a decreased particle diameter with a view to adding to surface area, however, the absorbent resin conversely suffers a decrease in the ratio of water absorption capacity under pressure or under no pressure because the decrease of particle diameter degrades the physical strengths of resin and contracts the gaps between the particles of resin swelled with absorbed water.
A technique is known which cross-links molecular chains near the surface of an absorbent resin and increases the cross-link density in the surface layer of the resin with a view to enabling the polymer, when swelled with absorbed water, to secure gaps for transfer of liquid between the particles thereof and consequently prevent the absorbent resin from decreasing the ratio of water absorption capacity even on exposure to pressure. Incidentally, the largeness of surface area and the ratio of water absorption capacity under pressure generally contradict each other such that the surface layer of resin is cross-linked uniformly based on difficulty of uniformity mixing of the resin and a cross-linking agent when the particles of resin have an increased surface area. Most of the improvements achieved to date with a view to heightening the ratio of water absorption capacity of an absorbent resin under pressure and increasing the speed of absorption thereof as well have stemmed from combinations of the two methods (increase of surface area and cross-linkage near surface) mentioned above. Regretfully, however, these improvements are incapable of yielding satisfactory results owing to the technical limit imposed by the fact that the two properties contradict each other as mentioned above.
As a means for breaking through this technical limit by enlarging the surface area without decreasing the particle diameter, the concept of obtaining a porous polymer by using a foaming agent and so on and the technique for giving a cross-linking treatment to the proximity of the surface of the polymer have been proposed in recent years. As concrete examples of this approach, a method which comprises polymerizing a 70 mol % neutralized aqueous sodium polyacrylate solution in the presence of a cross-linking agent and a carbonate as a foaming agent and cross-linking the proximity of the surface of the produced polymer (U.S. Pat. No. 5,314,420 and JP-A-07-185331), a method which comprises polymerizing a 70 mol % neutralized aqueous potassium polyacrylate or aqueous ammonium salt solution in the presence of a cross-linking agent and a carbonate salt as a foaming agent and a cross-linking the proximity of the surface of the produced polymer (WO 9502002, U.S. Pat. No. 5,712,316 and EP 707603), a method which comprises polymerizing a partially neutralized aqueous sodium acrylate solution in the presence of a solid foaming agent in a dispersed state and giving a cross-linking treatment to the proximity of the surface of the produced polymer (EP 744435), and besides a method using foaming agents a method which comprises obtaining a sodium acrylate polymer with a porous surface by the reverse-phase suspension polymerization performed in the presence of a specific surfactant and giving a cross-linking treatment to the proximity of the surface of the polymer (EP 0695762) may be cited.
Indeed, these methods which give a cross-linking treatment to the proximity of the surface of a porous polymer using a foaming agent have improved the ratio of water absorption capacity and the speed of absorption under pressure. In spite of the improvements, however, these methods still suffer degradation of the affinity of the surface of the polymer, typically porous in texture, for water because the porous polymer, on being subjected to the cross-linking treatment given to the proximity of the surface thereof, does not permit each control of the cross-link density owing to the polymer constitution itself the absorbent resin when the density of cross link in the surface is increased for the purpose of heightening the ratio of water absorption capacity under pressure. In spite of deliberate use of the porous polymer, therefore, there are problems of ultimate degradation of the speed of absorption.
It has been heretofore known to use such absorbent resin as exhibits a high ratio of water absorption capacity under pressure under load for disposable diapers. The loads proposed to date for use in the determination of a ratio of water absorption capacity, a factor typically representing physical properties fit for actual use of a relevant absorbent resin in diapers include 10 g/cm.sup.2 (JP-A-60-135432), 0.3 psi (about 20 g/cm.sup.2, EP 0339461), 50 g/cm.sup.2 (JP-A-63-99861), 63000 dynes/cm.sup.2 (about 50 g/cm.sup.2, WO 9511932, U.S. Pat. No. 5,684,106),60 g/cm.sup.2 (EP 707603), total of minimum 0.01 --maximum of 0.7 psi (total of about 0.7 g/cm.sup.2 --maximum of 63 g/cm.sup.2, U.S. Pat. No. 5,601,542), and 0.6 through 1 psi (about 43 g/cm.sup.2 through about 71 g/cm.sup.2, JP-A-08-510484, U.S. Pat. No. 5,629,377), for example. The absorbent resins which reflect these specific loads have been disclosed there. Among these conventional absorbent resins, those which exhibit the highest ratios of water absorption capacity under pressure are the absorbent resin manifesting a ratio of 25.9 g/g under a load of 1 psi (about 71 g/cm.sup.2) (Test Run 6e of JP-A- 09-501975, WO 9505856) and the absorbent resin manifesting a ratio of 8 ml/g under a load of 1.7 psi (about 121 g/cm.sup.2) (Table B of EP 0339461). While these absorbent resins manifest ratios of water absorption exceeding 30 g/g under relatively low pressure, these ratios of water absorption capacity under load notably fall generally as the load increases (U.S. Pat. No. 5,601,542, EP 0707603, U.S. Pat. No. 5,629,377, and Table B of EP 0339461). When the load increases from 0.3 psi to 0.6 psi, for example, the ratio of water absorption capacity under pressure in most cases falls notably to not more than one half. The resins under a load of 100 g/cm.sup.2 (about 1.4 psi) manifested a capacity for absorption of not more than 10 g/g, or not more than 20 g/g at most, even when the surface area of resin was increased for getting preeminently speed of absorption. Regretfully, the conventional techniques have not fully satisfied the needs of the market for an absorbent resin manifesting high ratio of water absorption capacity under pressure and fair speed of absorption under still higher pressure, i.e. conditions necessary for a disposable diaper of high performance. Thus, an absorbent resin which satisfactorily reconciles the two factors, i.e. ratio of water absorption capacity and speed of absorption, remains yet to be completed.
In the case of a disposable diaper, since a baby wearing one incessantly moves about and, therefore, exerts unsteady load thereon, there are times when even an absorbent resin manifesting a due ratio of water absorption capacity under high pressure fails to exhibit a stable capacity for absorption to pressure. Simply because an absorbent resin manifests a due ratio of water absorption capacity under a load of 10 to some tens of g/cm.sup.2 which is estimated from the standard body weight (about 10 kg) of a baby, when incorporated in an actual diaper, is suspected that the absorbent resin fails to bring about a fully satisfactory result in disposable diapers. When the ratio of water absorption capacity under pressure which has been determined by simply varying the load (10--some tens of g/cm.sup.2) as practiced heretofore is relied on to rate or screen a given absorbent resin, it does not serve as a reliable index for the performance of the absorbent resin such as in disposable diapers. For the purpose of final evaluation of an absorbent resin intended for use in a diaper, the determination of the absorbent resin for such physical properties as ratio of water absorption capacity under pressure is not satisfactory. As things now stand, it is after all necessary to spend cost, time, and labor copiously to manufacture diapers and subject them to a test by monitors.